Thin film transistors (TFTs) are widely used as a switching element for a display such as a liquid crystal display device. The representative TFT has a gate electrode, an insulating layer and an organic semiconductor layer on a substrate sequentially and has a source electrode and a drain electrode formed on the organic semiconductor layer with a predetermined distance. The organic semiconductor layer forms a channel region, and an electric current flowing between the source electrode and the drain electrode is controlled by applying a voltage to the gate electrode to allow the on/off action.
Heretofore, the TFT has been made of amorphous silicon or polycrystalline silicon. However, a CVD apparatus which is used for production of the TFT using such silicon is very expensive, and to enlarge a display apparatus or the like using TFTs had a problem of significant increase in the production costs. Further, the process of making the amorphous or polycrystalline silicon into a film is carried out at a very high temperature so that materials usable for a substrate are limited. Thus, there is a problem that a lightweight substrate such as a resin substrate cannot be used.
In order to solve the problems, a TFT using an organic substance (hereinafter often referred to as an “organic TFT”) in place of the amorphous or polycrystalline silicon has been proposed. As a film-forming methods used for production of a TFT with an organic substance, vacuum deposition, a coating method and the like are known. By the use of these film-forming methods, enlarging a device is feasible while suppressing an increase in the production cost. Further, the process temperature required at the time of film-forming can be kept to relatively low temperature. By this, the organic TFT has an advantage of small limitation in the selection of materials used for a substrate. Therefore, practical application of the organic TFT is expected, and extensive studies and reports are made.
As materials for a p-type FET (field effect transistor) of an organic semiconductor used for an organic TFT, polymers such as conjugated polymers and thiophene polymers, metal phthalocyanine compounds, fused aromatic hydrocarbons such as pentacene, and the like are used alone or in a mixture with other compounds. As materials for an n-type FET, for example, 1,4,5,8-naphthalenetetracarboxyl dianhydride (NTCDA), 11,11,12,12-tetracyanonaphtho-2,6-quinodimethane (TCNNQD), 1,4,5,8-naphthalenetetracarboxyldiimide (NTCDI) and fluorinated phthalocyanine are known.
On the other hand, as a device using electric conduction in the same manner, an organic electroluminescence (EL) device is known. In the organic EL device, a strong electric field of 105 V/cm or higher is applied to a very thin film typically having a thickness of 100 nm or less in the film thickness direction to compulsorily flow charges. On the other hand, in the case of the organic TFT, it is necessary to flow charges over distance of several μm or more at a high velocity in an electric field of 105 V/cm or lower. Therefore, the organic substance itself used for an organic TFT needs to have a higher conductivity. However, the above-mentioned conventional compounds used for an organic TFT have a small field effect mobility and slow response speed, thus, they have a problem that a high response speed required in a transistor cannot be obtained. Also, their on/off ratios are small.
Here, the “on/off ratio” means a value obtained by dividing an electric current flowing between the source-drain when applying a gate voltage (on) by an electric current flowing between the source-drain when applying no gate voltage (off). The on current usually means an electric current value (saturated electric current) at the time when an electric current flowing between the source-drain reaches saturation with a gradual increase of the gate voltage.
As for reasons for the small field effect mobility and slow response speed of the above-mentioned compounds, low planarity of the pi-conjugated structure in the compounds may be mentioned. Then, it is attempted to increase the planarity by introduction of a divalent vinyl residue or a divalent ethynyl residue to the pi-conjugated system.
For instance, Patent Document 1 discloses a compound having an arylethynylene group. In this document, a monolayer processing is applied to a substrate during fabrication of a device, and further deposition is conducted while heating the substrate to obtain a high mobility. However, there is a problem that the process is complicated because the monolayer processing is applied to a substrate and further deposition is conducted while heating the substrate.
It is expected that planarity of the pi-conjugated structure in the compound is increased by bonding a thiophene ring to the divalent vinyl residue or the divalent ethynyl residue. As the compound having such a structure, Patent Document 2 discloses that an ethynylthiophene compound exhibits the transistor property. However, the mobility is 3×10−4 cm2/Vs and is not so satisfactory.
[Patent Document 1] WO2006/113205
[Patent Document 2] JP-A-2004-179249
The invention is made to solve the above-mentioned problems, and provides a compound for an organic thin film transistor which has a high mobility. Further, an object of the invention is to provide an organic thin film transistor using it as an organic semiconductor layer.